lang: ast, gapi, interfaces, parser: Print line numbers on error

This adds an initial implementation of printing line numbers on type
unification errors. It also attempts to print a visual position
indicator for most scenarios.

This patch was started by Felix Frank and finished by James Shubin.

Co-authored-by: Felix Frank <Felix.Frank.de@gmail.com>

lang/unification/fastsolver/fastsolver.go

@@ -134,8 +134,15 @@ func (obj *FastInvariantSolver) Solve(ctx context.Context, data *unification.Dat
 		if err := unificationUtil.Unify(x.Expect, x.Actual); err != nil {
 			// Storing the Expr with this invariant is so that we
 			// can generate this more helpful error message here.
-			// TODO: Improve this error message!
-			return nil, errwrap.Wrapf(err, "unify error with: %s", x.Expr)
+			displayer, ok := x.Node.(interfaces.TextDisplayer)
+			if !ok {
+				obj.Logf("not displayable: %v\n", x.Node)
+				return nil, errwrap.Wrapf(err, "unify error with: %s", x.Expr)
+			}
+			if highlight := displayer.HighlightText(); highlight != "" {
+				obj.Logf("%s: %s", err.Error(), highlight)
+			}
+			return nil, fmt.Errorf("%s: %s", err.Error(), displayer.Byline())
 		}
 		if obj.Debug {
 			e1, e2 := unificationUtil.Extract(x.Expect), unificationUtil.Extract(x.Actual)

lang/unification/solvers/unification_test.go

@@ -859,6 +859,11 @@ func TestUnification1(t *testing.T) {
 
 			data := &interfaces.Data{
 				// TODO: add missing fields here if/when needed
+				Metadata: &interfaces.Metadata{},
+				SourceFinder: func(string) ([]byte, error) {
+					return nil, fmt.Errorf("not implemented")
+				},
+
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
 					t.Logf(fmt.Sprintf("test #%d", index)+": ast: "+format, v...)